1. Field of the Invention
The present invention relates to a method and apparatus for separation of signal components, one of which is modulating a main carrier and another of which is a quadrature modulated subcarrier which is interleaved between some adjacent sidebands of the main carrier on a portion of the spectrum thereof. More particularly, the method and apparatus of the present invention effectively applies adaptive comb filtering techniques to a quadrature modulated subcarrier color television signal in order to separate chrominance and luminance signal components without significant signal degradation due to resolution losses or chrominance to luminance cross-talk, effects which are generated by the previously known separation techniques.
2. Description of the Prior Art
In consideration of designs for color television systems and signal formats, it has been recognized that the human eye does not require as much chrominance (color hue and saturation) detail information as is required for luminance (brightness) information for the same apparent picture resolution. Since the eye is extremely sensitive to brightness changes insofar as resolution is concerned, most television systems signal formats, such as the National Television System Committee (NTSC) format, devote approximately 4 megahertz of bandwidth to luminance information (the NTSC standard being 4.2 megahertz). For hue or saturation changes which are not accompanied by corresponding changes in luminance, the eye is satisfied with one-third to one-tenth of the resolution needed for brightness. Thus, encoded color subcarrier signals are accorded much less bandwidth than that devoted to luminance information. In the NTSC system the I color subcarrier component has a 1.3 MHz bandwidth while the Q color subcarrier component bandwidth is only 500 KHz. The consequence of bandwidth limitations on color information subcarrier signals is that high frequency, i.e., minute picture details, are monochrome, and that the color information applies only to larger picture details.
To reduce visible dot interference in the television picture, the NTSC color subcarrier frequency of 3.579545 MHz was chosen so that its sidebands are at odd multiples of half of the line scanning frequency, which results in the color side bands being interleaved between high frequency side bands of the luminance signal. Thus, successive dots or picture elements resulting directly from the subcarrier on one scanning line are offset and interleaved with dots on the next scanning line. The interleaved pattern of dots resulting from the subcarrier has heretofore been generally accepted by most viewers of black and white receivers as a noticeable but unobjectionable grid pattern. Some of the reasons why the color subcarrier dot pattern have not been objected to or even seen are that the subcarrier is a high frequency, so that the dots appear very close together and are not therefore too visible at a distance from the picture screen. As already noted, the dots are interleaved from one scanning line to the next, which further reduces their visibility at a distance. Furthermore, most monochrome television receivers of domestic origin are characterized by a relatively low picture bandwidth and therefore will not even reproduce the dot pattern. Finally, since most information in a television picture is generally of low saturation and since it is rare to observe a fully saturated color image outside of a color bar test pattern, for example, the intensity of the dot pattern from the color subcarrier is not much greater than ambient luminance levels upon which the subcarrier dot pattern rides. It has been found that the average color saturation of a television picture is around 20%. Thus, the brightness of a subcarrier dot rarely exceeds 20% of the ambient luminance present on the scanning line.
With pictures displayed on color television receivers, the interleaving of chrominance side bands between luminance side bands at the upper end of the luminance channel spectrum works well for static and low saturation color picture information, where chrominance levels are not exceeding 20%. When high chrominance energy levels are present, the dot pattern levels in the luminance path are excessive and, combined with the high gamma of the picture tube are shifting upwards the average brightness of saturated areas. As a result, for example, a high saturation red will be displayed on the color screen as a pinkish color. It is therefore mandatory, in a color television display, to filter out the subcarrier from the luminance channel prior to application to the display to the display device.
The heretofore commonly employed expedient in color television receivers has been to install a band-reject filter (usually an LC trap) in the luminance path, with a low end 3db attenuation point being between 2.3 and 2.8 MHz. As a result an incoming transition with 150 nanoseconds rise time will be degraded to 250 to 300 nanoseconds. Thus, it is appreciated that heretofore there has been a significant and continual loss of picture resolution in most color television receivers. Also, the band-reject filter introduced ringing on trailing edges of high speed transitions and pulses such as those exemplified by the sine-square test pulse.
Another problem, encountered when imperfect chrominance-luminance separation techniques were used, resulted from the demodulation by the set chroma channels of luminance high frequency components, mistakenly taken as chrominance information by the television set demodulator. As a result, fast luminance transitions were often followed by a rainbow pattern when a simple band-pass filter was used to remove luminance components from the chrominance channel.
Comb filters have been known to separate interleaved components in a complex spectrum with minimum degradations.
As early as 1930, in U.S. Pat. No. 1,769,920, F. Gray described the utilization of a comb filter design to enable interleaving two distinct television picture signals into a single spectrum corresponding to the audio frequency range.
With the advent of the NTSC color television signal format, in which the color subcarrier is interleaved between the high frequency sideband components of the luminance signal, it was proposed to employ the Gray filtering technique to the separation of chrominance and luminance signal information in a color television receiver. Gray's technique was noted in U.S. Pat. No. 3,542,945 to N. W. Parker, issued Nov. 24, 1970. Parker implemented a form of comb filter by combining bandpassed composite video signal (chroma with interspersed luminance components) in an adder with a second input to the adder being bandpassed video first having been passed through a one horizontal scanning line delay line and a phase inverter. Since two successive lines of NTSC chroma subcarrier are 180.degree. out of phase by reference to horizontal synchronization pulses, the two inputs combined as a color subcarrier sum whereas since lines of luminance are originally in phase, the combination of the oppositely phased luminance components inputs to the adder resulted in cancellation therein, thereby providing a comb filtered chrominance output. Parker's circuit then added the separated chrominance without further phase reversal to the composite video input signal (luminance-plus-chrominance) wherein phase cancellation of chrominance occurred resulting in a separated luminance output. In static pictures having low chrominance levels, the Parker comb filter technique worked well. However, for sudden color changes along a vertical axis and other dynamic changes in chrominance signals within the television picture, Parker's comb filter did not eliminate subcarrier dot patterns and other highly visible aberrations and artifacts deviating from the true picture on account of momentary phase shift and consequent loss of interleaving of the color signal within the upper frequency portion of the luminance signal. Another weakness of this approach was the introduction of ringing and echoes in luminance transitions due to the imperfections of 1-line delay lines. These errors were visible even when the comb filter was not required, that is, when the picture color saturation was low.
The usefulness of comb filtering in specialized television applications such as in time base error correction within color video recording and playback systems was disclosed in two of the present applicant's prior U.S. Pat. Nos. 3,674,920 issued July 4, 1972, and 3,764,739 issued Oct. 9, 1973.
In an article entitled "Comb Filter Improvements With Spurious Chroma Deletion," published in SMPTE Journal Vol. 86, No. 1, January 1977, pages 1-5, Arthur Kaiser proposed a comb filter color decoder in which the comb filter was replaced during chroma transitions by a conventional low pass filter in the form of a trap. Kaiser's "adaptive" comb filter, while somewhat effective for its intended purpose, was not particularly effective during diagonal chroma transitions; and, implementation of Kaiser's circuitry as blocked out in FIG. 9 of his article, required two full bandwidth delay lines (4.2 MHz bandwidth modules which were then, and still are, very expensive). As a result, Kaiser's circuitry was practically limited to those situations, such as in television studios, where demanding video requirements may have justified the high cost of implementing his chrominance and luminance separation scheme. Another defect, known as "scalloping," was due to the presence of light vertical lines during chroma horizontal or oblical transitions, and was created by the use of two delay lines and perfect vertical alignment of chroma combing residues from line to line.